1. Field of the Invention
The present invention relates to a rangefinder apparatus for measuring the distance to an object to be measured; and, in particular, to an active type rangefinder apparatus suitably used in a camera or the like.
2. Related Background Art
In active type rangefinder apparatus used in cameras and the like, an infrared light-emitting diode (IRED) projects a luminous flux toward an object to be measured, the reflected light of thus projected luminous flux is received by a position sensitive detector (PSD), a signal outputted from the PSD is arithmetically processed by a signal processing circuit and an arithmetic circuit and then is outputted as distance information, and the distance to the object is detected by a central processing unit (CPU). In general, since errors may occur when the distance is measured upon a single light-projecting operation, light is projected a plurality of times so as to obtain a plurality of distance information items, which are then integrated by an integrating circuit and averaged.
As such an active type rangefinder apparatus, one shown in FIG. 1 has conventionally been known. FIG. 1 is a configurational view of a rangefinder apparatus in accordance with first prior art.
In the rangefinder apparatus shown in this drawing, a CPU 1 controls individual elements according to a program stored in an electrically erasable and programmable read-only memory (EEPROM) 2 and, under the control of the CPU 1, a driver 3 drives an IRED 4 so as to make it output infrared light, which is then projected through a projection lens 101 to an object to be measured. The infrared light reflected by the object is collected by a PSD 5 by way of a light-receiving lens 102, and the PSD 5 outputs two signals I.sub.1 and I.sub.2 according to the position at which the reflected light of the infrared light is received. A first signal processing circuit 11 eliminates a steady-state light component contained in the signal I.sub.1 which becomes a noise, whereas a second signal processing circuit 12 eliminates a steady-state light component contained in the signal I.sub.2 which becomes a noise.
According to the signals I.sub.1 and I.sub.2 from which the steady-state light components have been eliminated, an arithmetic circuit 14 determines an output ratio (I.sub.1 /(I.sub.1 +I.sub.2)) by an arithmetic operation, and outputs an output ratio signal corresponding to the distance to the object. An integrating circuit 15 integrates at an integrating capacitor 6 the output ratio signals thus outputted from the arithmetic circuit 14 a plurality of times, thereby improving the S/N ratio. The signal outputted from this integrating circuit 15 (hereinafter referred to as "AF signal") corresponds to the distance to the object. Then, according to the AF signal outputted from the integrating circuit 15, the CPU 1 determines a distance signal by carrying out a predetermined arithmetic operation, and controls a lens driving circuit 7 according to this distance signal, so as to move a lens 8 to an in-focus position.
FIG. 2 is a graph showing the relationship between the AF signal outputted from the integrating circuit 15 in this first prior art and the distance to the object. In this graph, the abscissa indicates the reciprocal (1/L) of the distance L to the object, whereas the ordinate indicates the output ratio (I.sub.1 /(I.sub.1 +I.sub.2)), i.e., AF signal. As shown in this graph, the output ratio has substantially a linear relationship with respect to the reciprocal (1/L) of the distance L at a certain distance L.sub.4 or less, such that the output ratio decreases as the distance L is longer (1/L is smaller). At the distance L.sub.4 or greater, by contrast, the influence of the steady-state light component increases as the distance L is greater. Letting I.sub.n (I.sub.n.gtoreq.0) be the steady-state light component, the output ratio is (I.sub.1 +I.sub.n)/(I.sub.1 +I.sub.n +I.sub.2 +I.sub.n) whereby the output ratio would shift so as to increase at the distance L.sub.4 or greater. Also, since I.sub.n occurs randomly, it becomes unstable depending on the distance measuring condition. It is due to the fact that, as the distance L increases, the intensity of reflected light received by the PSD 5 decreases, whereby the steady-state light component I.sub.n becomes relatively greater. If such a phenomenon occurs, the distance to the object L cannot be determined uniquely from the output ratio.
Therefore, as a rangefinder apparatus overcoming such a problem, one which will be explained in the following has been known. FIG. 3 is a configurational view of the rangefinder apparatus in accordance with second prior art. In the rangefinder apparatus in accordance with the second prior art, a clamping circuit 13 is disposed between the second signal processing circuit 12 and arithmetic circuit 14 in the rangefinder apparatus in accordance with the first prior art. The clamping circuit 13 inputs therein the signal I.sub.2 outputted from the second signal processing circuit 12, compares a clamp signal I.sub.c having a certain constant level and the signal I.sub.2 with each other in terms of magnitude of their levels, and outputs the clamp signal I.sub.c if the former is greater and outputs the signal I.sub.2 as it is if not. Namely, the arithmetic circuit 14 outputs the output ratios (I.sub.1 /(I.sub.1 +I.sub.2) and (I.sub.1 /(I.sub.1 +I.sub.c) when the distance L to the object is shorter and longer, respectively.
FIG. 4 is a graph showing the relationship between the AF signal outputted from the integrating circuit 15 in this second prior art and the distance to the object. In this graph, the abscissa indicates the reciprocal (1/L) of the distance L to the object, whereas the ordinate indicates the output ratio, i.e., AF signal. As shown in this graph, in each of the respective ranges smaller and greater than a certain distance L.sub.4, the output ratio has substantially a linear relationship with respect to the reciprocal (1/L) of the distance L, such that the output ratio decreases as the distance L is greater(1/L is smaller). As a consequence, the distance L to the object can be determined uniquely from the output ratio.